This invention relates to a solid electrolyte capacitor, and more particularly to a cathode structure thereof.
In general, a solid electrolyte capacitor element comprises a sintered porous anode body of a valve-metal such as tantalum, an anode wire of the same valve-metal partly embedded into the anode body, an oxide film of the valve-metal covering the surfaces of the anode body and the anode wire, a solid electrolyte layer of, for instance, manganese dioxide (MnO.sub.2) overlaying the oxide film, and a cathode layer coated on the solid electrolyte layer. The cathode layer includes a graphite layer provided on the solid electrolyte layer, a plated metal layer of copper, nickel or another highly conductive metal, and a solder layer successively formed on the graphite layer. The plated metal layer is used in place of a silver paste layer which is expensive and presents problems in that the silver paste tends to be melted by the heat generated when the solder layer is formed in order to cause a so-called silver leaching phenomenon. That is, silver particles are diffused into a solder bath, thereby causing an exfoliation of the solder layer and/or increase in tangent of dielectric loss (referred to as "tan" .delta. herein). Permeating moisture ionizes the silver paste and silver ions reach the oxide film to cause a chemical reaction, which often damages the oxide film to increase linkage current. The plated metal layer is formed by the electroless plating technique.
Japanese patent application laid-open under No. Sho 59-63716 teaches a cathode layer which comprises a first graphite layer coated on the solid electrolyte layer to reduce the equivalent series resistance of the anode body. A second graphite layer contains thermal resistant resin to prevent a penetration of the plating solution into the solid electrolyte layer and formed on the first graphite layer, and a plated nickel layer. Since the normal graphite layer, such as the first graphite layer, is not enough to prevent the plating solution from penetrating onto the solid electrolyte layer, the second graphite layer is effective to prevent a degradation of the electrical properties of the capacitor. However, it is extremely difficult to perform the electroless plating on the second graphite layer containing graphite and thermal resistant resin to form the plated metal layer thereon. Though it is known that the graphite-coated capacitor element is immersed in an aqueous solution of stannous chloride, and then in an aqueous solution of palladium chloride to physically adsorb palladium particles on the graphite layer for activation of the graphite surface for the electroless plating of copper, nickel, etc., palladium particles are merely physically adsorbed on the graphite surface in this method. The adhesion of the formed plated layer to the graphite layer is extremely weak to cause an exfoliation of the plated layer by the thermal stress generated when the solder layer is formed upon the plated layer.
Another problem of the capacitor structure described by the above-mentioned Application is associated with a water-repellent insulating resin coating provided over the oxide film around the root of an anode wire, which is formed before the formation of the solid electrolyte layer for preventing direct contact between the oxide film and the plating solution.
As the insulating resin coating is water-repellent, however, the solid electrolyte layer of manganese dioxide is not formed on a peripheral portion of the insulating resin coating to expose the oxide film there because the manganese dioxide is pyrolyzed from the manganese nitrate solution. Under such a condition, the first graphite layer, the second graphite layer and the plated layer are formed to make contact with the insulating resin coating. The result is that either one of these cathode conductive layers comes into contact with the oxide film without intervention of the manganese dioxide layer to drastically reduce the reliability and/or increase the initial leakage current.
An object of this invention is to provide a solid electrolyte capacitor with an improved cathode structure.
Another object of this invention is to provide a solid electrolyte capacitor having a graphite layer containing a thermal resistant resin in which a plated metal layer adheres strongly to the graphite layer.
Still other object of this invention is to provide a solid electrolyte capacitor having a water-repellent insulating resin coating around the root of the anode wire which prevents a direct contact of the cathode layer to the oxide film.